Sensitivity and Overheads

The NIRI Integration Time Calculator (ITC) can be used to determine limiting magnitudes, exposure times, S/N ratios, background levels, etc. for a wide range of source properties, observing conditions, and NIRI configurations. It is used both for (a) NIRI standalone imaging and spectroscopy and (b) for NIRI with adaptive optics (i.e., ALTAIR) at present in imaging mode only, either using a natural guide star or the laser guide star). The ITC is divided into five sections: (1) Spatial profile and brightness of source; (2) Spectral distribution of source; (3) Instrument and telescope configuration; (4) Observing conditions; and (5) Details of observation. It is in the third of these sections that the user selects or de-selects adaptive optics.

The ITC is not available for AO imaging at wavelengths beyond the K band or for AO spectroscopy. However, it is possible to use ALTAIR for imaging in the 3-4μm region and for JHK spectroscopy.

For AO imaging at L', throughput is reduced by a factor of about 7 because the Altair dichroic was not originally designed for thermal observations. Experiments show that with ALTAIR the background per pixel at L' increases by 10X (3.2X increase in noise). Thus, signal to noise (S/N) is reduced by a factor of ~20 with ALTAIR due to the dichroic and warm optics. ALTAIR does improve a 0.3" FWHM image of a point source at L' to ~0.1" FWHM, implying a S/N increase of 9. Thus, overall, there is about a factor 2 reduction in S/N with the use of ALTAIR in L'. However, there is an overall gain in angular resolution, which may be useful for some programs involving bright targets which require improved angular resolution.

For AO spectroscopy one can obtain a rough and conservative estimate by using the NIRI ITC with the f/6 camera, the narrowest (2 pixel-wide) slits, best image quality (IQ20) and much shorter individual exposures than planned for the f/32 observations. Note that the f/6 result will be misleading if the same individual exposure times are used at f/32. The background per pixel is 25 times less at f/32, and thus individual exposure times need to be 25 times longer at f/32 than at f/6 to produce the same background (the same noise level) and to achieve the same S/N in the same total exposure time. For example, to estimate the S/N in a one hour integration consisting of twelve 300-second exposures for f/32 spectroscopy, specify three-hundred 12-second exposures in the f/6 NIRI ITC.

The ITC has been used to generate tables of sensitivity estimates for both imaging and spectroscopy, which usually agree with values measured on the telescope to 20%. However, because some conditions vary unpredictably (e.g., the OH sky emission, which dominates the 1-2.4μm background, can vary by a factor of two from hour to hour), one should consider the ITC predictions to be accurate only to within a few tens of percent.